patch 9.1.1819: Cannot configure the inner foldlevel indicator

Problem:  Cannot configure the inner foldlevel indicator for the
          foldcolumn
Solution: Add "foldinner" suboption value to the 'fillchar' option
          (Maria José Solano).

closes: #18365

Signed-off-by: Maria José Solano <majosolano99@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/fold.txt

@@ -1,4 +1,4 @@
-*fold.txt*      For Vim version 9.1.  Last change: 2025 Jul 15
+*fold.txt*      For Vim version 9.1.  Last change: 2025 Oct 03
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -595,7 +595,8 @@ A closed fold is indicated with a '+'.
 These characters can be changed with the 'fillchars' option.
 
 Where the fold column is too narrow to display all nested folds, digits are
-shown to indicate the nesting level.
+shown to indicate the nesting level.  To override this behavior you can use
+the "foldinner" character of the 'fillchars' option.
 
 The mouse can also be used to open and close folds by clicking in the
 fold column:

runtime/doc/options.txt

@@ -3853,6 +3853,9 @@ A jump table for the options with a short description can be found at |Q_op|.
 	  foldopen	'-'		mark the beginning of a fold
 	  foldclose	'+'		show a closed fold
 	  foldsep	'|'		open fold middle character
+	  foldinner	none		character to show instead of the
+					numeric foldlevel when it would be
+					repeated in a narrow 'foldcolumn'
 	  diff		'-'		deleted lines of the 'diff' option
 	  eob		'~'		empty lines below the end of a buffer
 	  lastline	'@'		'display' contains lastline/truncate

runtime/doc/version9.txt

@@ -1,4 +1,4 @@
-*version9.txt*  For Vim version 9.1.  Last change: 2025 Sep 29
+*version9.txt*  For Vim version 9.1.  Last change: 2025 Oct 03
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -322,6 +322,8 @@ Improvements in 'fillchars':
   "eob" in 'fillchars'.
 - Support for using multibyte items with the "stl", "stlnc", "foldopen",
   "foldclose" and "foldsep" items in the 'fillchars' option.
+- Support for configuring the character used inside a fold level region using
+  "foldinner" in 'fillchars'.
 
 Support for the XChaCha20 encryption method. 'cryptmethod'
 
@@ -41702,7 +41704,9 @@ Options: ~
 - new option values for 'fillchars':
 	"trunc"		- configure truncation indicator, 'pummaxwidth'
 	"truncrl"	- like "trunc" but in 'rl' mode, 'pummaxwidth'
-	"tpl_vert"	- separators for the 'tabpanel'
+	"tpl_vert"	- vertical separators for the 'tabpanel'
+	"foldinner"	- character used inside the 'foldcolumn' for nested
+			  fold levels
 - 'grepformat' is now a |global-local| option.
 - adjust for GTK3 dropping some mouse cursors 'mouseshape'
 - 'nrformats' accepts the new "blank" suboption, to determine a signed or

src/screen.c

@@ -287,6 +287,8 @@ fill_foldcolumn(
 	    symbol = wp->w_fill_chars.foldopen;
 	else if (first_level == 1)
 	    symbol = wp->w_fill_chars.foldsep;
+	else if (wp->w_fill_chars.foldinner != NUL)
+	    symbol = wp->w_fill_chars.foldinner;
 	else if (first_level + i <= 9)
 	    symbol = '0' + first_level + i;
 	else
@@ -4738,6 +4740,7 @@ static struct charstab filltab[] =
     CHARSTAB_ENTRY(&fill_chars.foldopen,    "foldopen"),
     CHARSTAB_ENTRY(&fill_chars.foldclosed,  "foldclose"),
     CHARSTAB_ENTRY(&fill_chars.foldsep,	    "foldsep"),
+    CHARSTAB_ENTRY(&fill_chars.foldinner,   "foldinner"),
     CHARSTAB_ENTRY(&fill_chars.diff,	    "diff"),
     CHARSTAB_ENTRY(&fill_chars.eob,	    "eob"),
     CHARSTAB_ENTRY(&fill_chars.lastline,    "lastline"),
@@ -4856,6 +4859,7 @@ set_chars_option(win_T *wp, char_u *value, int is_listchars, int apply,
 		fill_chars.foldopen = '-';
 		fill_chars.foldclosed = '+';
 		fill_chars.foldsep = '|';
+		fill_chars.foldinner = NUL;
 		fill_chars.diff = '-';
 		fill_chars.eob = '~';
 		fill_chars.lastline = '@';

src/structs.h

@@ -3913,6 +3913,7 @@ typedef struct
     int	foldopen;
     int	foldclosed;
     int	foldsep;
+    int	foldinner;
     int	diff;
     int	eob;
     int	lastline;

src/testdir/test_display.vim

@@ -340,6 +340,32 @@ func Test_fold_fillchars()
         \ ]
   call assert_equal(expected, lines)
 
+  " check setting foldinner
+  set fdc=1 foldmethod=indent foldlevel=10
+  call setline(1, ['one', '	two', '	two', '		three', '		three', 'four'])
+  let lines = ScreenLines([1, 6], 22)
+  let expected = [
+        \ ' one                  ',
+        \ '[        two          ',
+        \ '-        two          ',
+        \ '[                three',
+        \ '2                three',
+        \ ' four                 ',
+        \ ]
+  call assert_equal(expected, lines)
+
+  set fillchars+=foldinner:\ 
+  let lines = ScreenLines([1, 6], 22)
+  let expected = [
+        \ ' one                  ',
+        \ '[        two          ',
+        \ '-        two          ',
+        \ '[                three',
+        \ '                 three',
+        \ ' four                 ',
+        \ ]
+  call assert_equal(expected, lines)
+
   %bw!
   set fillchars& fdc& foldmethod& foldenable&
 endfunc

src/version.c

@@ -729,6 +729,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1819,
 /**/
     1818,
 /**/